Field of the Invention
The present invention is generally directed toward the field of printed circuit board technology and, more particularly, toward a method for producing micro-holes in printed circuit boards.
In modern printed circuit board technology, increasingly micro-holes or micro-vias are being utilized, because the conventional mechanically drilled Z-axis connections take up too much space. There are a number of production methods for micro-holes in semi-finished components for printed circuit boards. Apart from plasma drilling, also laser drilling or photo-chemical structuring can be utilized for producing small holes in photosensitive dielectric materials.
While laser drilling is a sequential process, which in the case of large numbers of holes naturally also takes a long time, plasma drilling and also the method of photo-structured micro-holes are parallel processes, which enable all holes to be produced simultaneously.
Photo-chemical structuring manifests a number of disadvantages, in particular the impeccable application of liquid, photosensitive dielectric layers is very expensive and for this reason it is also very difficult to achieve a high production yield.
Plasma drilling is well known and, since 1991, has proved to be very successful. Nevertheless, there are some points, which are still worthy of improvement:
On the one hand, plasma drilling is a parallel process, i.e., all holes are produced simultaneously. The process lasts between 10 and 45 minutes, depending on the material and its thickness. The drilling speed can hardly be increased anymore for a given dielectric material and, particularly when a smaller number of holes are to be drilled, the method is not as economical as in the case of great hole densities. For this reason, it would be advantageous to increase the speed of plasma drilling still further so as to shorten the process time.
The plasma drilling process utilized is an isotropic etching process, which does not only act in the Z-direction of the semi-finished components, but also laterally, in the X-Y-direction. As a result of this, a so-called undercutting is produced, which in particular in the case of blind or pocket holes has to be eliminated by a further process, namely hole forming. In doing so, a layer of electrically conductive material is homogeneously removed and, simultaneously, overhanging edges of material is etched off. The disadvantages of this process are, on the one hand, the additional costs, and, on the other hand, an enlargement of the etched holes. It would be advantageous to prevent this undesirable undercutting.
Prior to the actual plasma drilling, in a photo-process all those points are made free of any conductive material, at which a hole is to be etched. This photo-structuring process cannot be carried out without any loss in yield. In particular, small dust particles can lead to the consequence that undesirable additional openings are produced in the electrically conductive material, which in the plasma also become holes, and which subsequently in the electric through-plating can lead to electric short-circuits.
The present invention presented is directed toward a method for the production of micro-holes that avoids or eliminates the disadvantages in the art described above. The inventive method is inexpensive, easy to apply and compatible with customary techniques of the printed circuit board industry.
In accordance with the present invention, a combination of an embossing process with subsequent cleaning of the holes is utilized. Embossing dies and embossing devices of this type are cheap to manufacture with conventional methods and are used for the production of apertures in the dielectric material. They can, however, also be utilized for the removal of local areas of electrically conductive material. In a secondary treatment, the embossed micro-holes are cleaned and subsequently electrically contacted through with the conventional processes of printed circuit board technology. Embossing of micro-holes according to the present invention results in a significant shortening of the process times and enables novel, very advantageous possibilities.